This invention relates to a method of constructing nonlinear chromatic transforms for improving the display or making color images on a particular medium from color digital images.
In the process of capturing digital images and viewing prints from such digital images, flare is introduced which degrades the viewed image in at least two ways. The first is by ambient light at the time of capture, and the second is by ambient light at the time of viewing of the print or display. In a typical silver halide photographic process, flare takes place in three ways. The first is at the time of exposure. The second is when the optical print is made. The third is when the optical print is viewed.
There have been a number of techniques for improving the tone reproduction of digital images, see for example, U.S. Pat. Nos. 4,792,518 and 5,300,381. For a discussion of tone reproduction and the effect of flare on print quality, see xe2x80x9cThe Tone Reproduction of Colour Photographic Materials,xe2x80x9d R. W. G. Hunt, I. T. Pitt, and P. C. Ward, J. Photog. Sci., 17:198(1969). Flare light is generally ambient light, which degrades the viewed image on the print.
As set forth in the above disclosures, a number of techniques are disclosed which require that the media be silver halide photographic media and the techniques for making the images are very complex. The publication by Hunt et al describes the xe2x80x9cideal systemxe2x80x9d for printing photographic images to correct for camera flare, printer flare, and viewing flare but offered no practical way to implement this theoretical tone reproduction curve because of the lack of digital imaging tools and the limitations of the materials available in 1969.
It has been the practice, in dealing with tone scale improvements, to treat only one tone scale, which includes all colors along with black and white information. One of the problems of changing the tone reproduction of digital images to reduce the loss in image quality from viewing flare is that sometimes shadow detail is lost because all of the colors along with black and white digital data are changed equally.
It is an object of the present invention to construct a transform for a predetermined medium for producing improved color images on a display or printer or on a particular medium which overcomes image degradation by viewing flare.
It is another object of the present invention to provide transforms for improving the image quality of digital images on displays with a wider dynamic range than previously available.
These objects are achieved by a method for constructing a nonlinear transform for a predetermined display or printer, such printer or display being adapted to form a color image, on a particular medium, the input to the transform including an input color digital image produced by a particular image capture device, the method comprising the steps of:
a) providing a luminance reproduction curve in response to the input color digital image which, for any input luminance value, will produce an output luminance value, the luminance reproduction curve being selected to preserve highlight and midtone contrast, and shadow detail from the input color digital image;
b) providing a plurality of chromatic reproduction curves, which are luminance independent, in response to the input color digital image which, for any input chroma value, will produce an output chroma value; and
c) the chromatic reproduction curves being selected so that each curve is a function of the visual density range of the display or medium and the image chroma values, wherein small chroma values remain unchanged and large chroma values are modified to increase or decrease chroma values while maintaining existing luminance values for a particular device or medium whereby a user can select one or more of the chromatic reproduction curves which, in combination with the luminance reproduction curve, produces color digital images so that a selected printer or display can form a color image, on a particular medium, with modified chroma values.
By substantially independently changing the tone scale of chroma data separately from luminance data, shadow detail may be preserved yet still realize the desirable effect of an increase in contrast of the chroma digital data which offsets viewing flare.
A feature of the present invention is that transforms are produced which can used by a printer or display for forming a color image on a particular medium with improved multiple chromatic reproductions, ensuring that such images provide esthetically pleasing results which facilitate better utilization of the dynamic range of the print media while protecting shadow detail from the effects of viewing flare.
The present invention solves a flare problem by a method, which has not been available to optical printing of silver halide images. Flare can be a function of ambient light which takes place at the time of camera exposure and as well as ambient light when viewing the print or display. It is therefore an advantage of the present invention to produce a set of transforms which can be used either in soft displays for displaying images, or for use in printing black and white or colored images and which minimizes the effects of flare and permits a better utilization to the dynamic range of the print media. These transforms can be readily implemented in a computing system. Such a computing system can include a microprocessor, which in combination with a digital printer, can produce continuous tone prints which are esthetically pleasing.
Heretofore, in silver halide reproduction optical printers, generally both luminance and chrominance are adjusted simultaneously to change the color of an image to be printed resulting in an increase in contrast which changes both luminance and chrominance simultaneously. The present invention recognizes that in digital image processing luminance and chrominance can be independently adjusted to preserve shadow detail and improved colorfullness thereby producing more esthetically pleasing color images.
Organic electroluminescent devices such as organic light emitting diode (OLED) displays represent a new technology for producing displays and the present invention is particularly suitable for use in constructing transforms for such displays.